Method A

  1. Touch and select the perimeter of a circle.
  2. Drag the perimeter over the location for attachment (i.e., Point b in the following image) and pause for the popup to offer attachment options.
  3. Moving the line or moving Point b will result in the radius of the circle moving. In addition, moving the center of the circle Point a in the example below will also change the radius of the circle.

Interactive Geometry - Example - Attach Circle Perimeter to Object